Placental
mammals, including humans, have a complex reproductive system. After
fertilization, the embryo develops inside the mother's uterus, attached to a
placenta that provides nutrients and removes waste. The placenta also acts as a
barrier, protecting the developing fetus from the mother's immune system. The
fetus undergoes various stages of development until it is ready for birth,
after which the placenta and other fetal tissues are expelled from the mother's
body. The process of placental mammal reproduction is essential for the
continuation of the species.
The
highest order of protection of the offspring is seen in man. It is evident from
the internal development, birth and later care and feeding of the young.
Male Reproductive
System
The
reproductive function of the male is the production of sperm cells and their
transfer to the female. Male reproductive system consists of:
(a)
Testes
(b)
Accessory Glands
(c)
Penis
Testes
The
paired testes (singular, testis) are formed inside the body cavity, but they
descend shortly before birth into an external sac between the thigh, the
scrotum. The testes contain about 15 to 20 coiled seminiferous tubules which
produce sperm cells or spermatozoa at a daily rate of a billion or so in young
man. The actual combined length of the seminiferous tubules in both testes is
about 225 meters.
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| Male Reproductive System |
Accessory
Ducts and Glands
Several structures are
associated with the testes. Applied directly to each testis is an epididymis
(Greek: upon the twins). It is a coiled tube that would be about six meters
long if it were stretched. The haploid sperm cells are stored in the
epididymis.
All the seminiferous
tubules of the testis lead into the epididymis, which then fuses into the major
seminal duct, the vas deferens (or ductus deferens). The vas deferens receives
secretions from the seminal vesicles and the prostate gland as it passes
alongside them to the urethra.
Penis
The urethra is the final
tube that leads through the penis and transports sperm outside the male body.
During ejaculation, fluids from the seminal vesicles and the prostate gland are
secreted into the deferens, and these secretions plus the sperm cells make up
the semen. Seminal fluid enhances sperm motility in various ways.
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| Testis And Sperm |
With the onset of sexual
excitement, the Cowper's glands secrete an alkaline fluid into the urethra to
neutralize the acidity of any remaining urine. These fluids also act as a
lubricant within the urethra to facilitate the ejaculation of semen. The
function of the penis is twofold. It carries urine through the urethra to the
outside, and it transports semen through the urethra during ejaculation.
Hormonal
Regulation in Males
Among the seminiferous
tubules in the testes are small masses of interstitial cells that secrete male
sex hormones, especially testosterone. Testosterone affects (1) the production of sperm cells (2) the development of the sex organs (3) the appearance of secondary male
sex characters.
FSH and LH produced by the
pituitary gland are chiefly responsible for the stimulation of spermatogenesis
and testosterone secretion in the testes.
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| Human Female Reproductive System |
Human Female Reproduction System
Sexuality in females is
more complex than it is in males. Not only do females have to produce eggs, but
after fertilization, they must nourish, carry and protect the developing
embryo. They must also nourish it for a time after it is born. Besides all
these, females have a monthly menstrual cycle. The female reproductive system
consists of the ovaries, ducts, and external genitalia.
The egg-producing organs
are the paired ovaries, elongated
bodies, about 5cm long at maturity. Ovaries have two functions: (1) they
release eggs (ova) fairly on a regular monthly schedule and (2) they produce
female sex hormones. The ovaries contain follicles, the actual center of egg
production. Each follicle contains a potential ovum, and follicles are always
present in several stages of development.
Each ovary is partially
covered by a funnel-shaped infundibulum, into which the egg passes when it is
released from the ovary. From the infundibulum, an egg can pass down the
fallopian tube (or oviduct) which leads to the uterus.
The uterus is a pear-shaped and sized organ located directly behind the
urinary bladder. The uterus leads downward to the vagina, a muscle-lined tube
about 8-10 cm long. A fold of skin called the hymen partially blocks the
vaginal entrance. The vaginal canal ends at the cervix or the neck of the
uterus.
Fertilization
of the ovum usually takes place in the fallopian tube or oviduct, near the
ovary. The zygote enters the uterus, and it is implanted in the wall of the
uterus and undergoes further development.
The embryo establishes a close contact with the uterine tissue through
which exchange of materials takes place. This tissue is called placenta.
Together with the amnion, the chorion helps to form the placenta.
The gestation period, called pregnancy, is usually 280 days in human
females. The edges of the amniotic folds come together around the stalk of the
amnion and yolk sac and form a tube, which leads from the embryo to the
placenta. This tube is called the umbilical cord. It contains the very
important blood vessels that connect the embryo with the placenta.
During pregnancy, the
uterus usually increases about 24 times. The process of childbirth or labor is
started and controlled by a variety of hormonal events. Birth has been found to
be brought about by a change in hormones produced by the placenta and also by a
hormone produced by the pituitary glands. Oxytocin brings about the
contractions of the uterus. The process of childbirth or parturition is
commonly divided into three stages: (I)
DILATION (II) EXPULSION (III) PLACENTAL STAGE OR AFTERBIRTH.
During the first stage of
childbirth, the cervix dilates or
becomes larger. This is the longest stage of childbirth. The opening of the
cervix is initially quite small. It must enlarge from a diameter of one to two
centimeters to 10 to 11 cm before the fetus can leave the uterus.
Dilation begins when the
muscles of the uterus begin to contract. These muscle actions of the uterus
gradually force the fetus and amniotic sac downwards. At first, the
contractions are mild and occur at 20 minutes apart. Later, the contractions
become stronger and may occur as close as a minute apart. Often towards the end
of this stage, the amniotic sac breaks releasing the amniotic fluid.
Expulsion:
The expulsion stage begins when the cervix is completely dilated and ends with
the birth of the baby. This stage is characterized by strong and frequent
contractions of the uterus. The expulsion of the fetus from the uterus to the
outside of the mother's body usually takes from half an hour to two hours.
Placental
stage: In the last stage of birth, also known as after-birth,
contractions of the uterus dislodge the placenta from the wall of the uterus.
Continued contractions expel blood, other tissue, fluids, and the placenta
through the vagina. The placenta is squeezed as it passes out of the body. This
action forces blood from the placental blood vessels into the body of the
newborn baby. After the umbilical cord stops pulsating, the cord is tied, and
then cut by a doctor. The placental stage is from ten to fifteen minutes.
Usually, at about the
ninth month after the conception
near the end of the human gestation period, the head is turned downwards
towards the vagina. At birth, therefore, the head usually comes out first.
However, very rarely the feet may appear first, causing complications. This
results in breech birth. In other
types of severe complications, for example, if the baby has been found to have
a weak heart, cesarean section may be performed in which the abdominal and
uterine wall is cut to remove the baby.
Placental
remains: The contraction and relaxation movements of the uterus
continue for some time after the birth. As a result, the placental remains are
expelled out.
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| The three stages of birth |
Mammary Glands
There is a pair of mammary
glands. They secrete milk and are accessory organs to the female reproductive
system. They are converging to the nipple and opening on its surface in large
numbers. The lymph nodes consist of many lobes. The lobes are divided into
lobules.
During pregnancy,
secretions of estrogen and progesterone cause the lobules to develop. The
actual milk-producing hormone is prolactin.
Milk:
It is produced by the mammary gland just at the time of childbirth. It contains
a special lymph-like fluid known as colostrum. It is rich in antibodies. The
true milk is not secreted until about the third day after childbirth. Usually,
the baby is fed on the mother's milk up to two years. The baby may be fed on
other sources of milk if required. As soon as the mother stops feeding the
baby, her reproductive cycle begins again. Sometimes the reproductive cycles
initiate even when the mother is breastfeeding.
Hormonal Control of the Female Reproductive Tract
Estrus:
For the proper timing of internal fertilization, in many animals, the female
informs the male that she is ready to mate. When the female is in estrus, or
“heat” (becomes sexually receptive to the male), she undergoes physical and
behavioral changes. These changes coincide with the maturation of eggs within
the ovary.
Estrous
Cycle: The physiological changes occurring from one period of
estrus to the next are called the estrous cycle. It is controlled by hormones.
The timing of the estrous cycle varies in different animals, e.g., rats: 5
days, bitch: every 6 months. The estrous cycle occurs in many animals. Example:
rats, mice, cats, dogs.
Menstruation:
The periodic shedding of blood and tissue from the uterine lining, the
endometrium, is called menstruation.
Menstrual
cycle: Unlike many other mammals, eggs of the human female
mature quite often. Human females do not undergo a seasonal estrous cycle as
dogs or cats do. About once every twenty-eight days, a mature egg is released
from an ovary. At the same time, certain uterine changes occur to prepare for a
possible pregnancy. This monthly egg maturation and uterine preparation is
called the menstrual cycle.
The Role of the Ovary
The ovary consists of two
types of cells: (a) germ cells (b) somatic cells. Germ cells give rise to the
ova (eggs). Somatic cells are the follicle cells.
Follicle
Cells: These are concerned with the support and nutrition of
the developing ova. The follicle cells secrete estrogen. It is a female sex
hormone. Estrogen acts on the uterus. It causes the endometrium of the uterus
to divide rapidly. Some of the cells of the endometrium become glandular in
nature. They start secreting proteinaceous secretions. When the embryo reaches
the uterus, it starts taking up glandular type proteinaceous secretion. The
wall of the uterus becomes thick, and more blood vessels develop in it due to
estrogen."
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| Ovary and follicles |
The ovum is surrounded by
a follicle. The ovulation takes place, and as a result, the follicle ruptures.
After rupture, the follicle is converted into a mass of specialized tissue
called corpus luteum. If the discharged ovum is fertilized, the corpus luteum
remains until the placenta is formed later. It gradually turns into scar
tissue. If the ovum is not fertilized, the corpus luteum degenerates and turns
into scar tissue. The corpus luteum is cream in color (corpus luteum means
yellow body). Its cells become glandular, and they secrete a hormone called
progesterone.
Functions
of Progesterone: It prepares the myometrium, endometrium,
and muscles of the uterine wall for the implantation of zygote.
Menstrual
Cycle: There is periodic shedding of blood and tissue from
the endometrium of the uterus. It is called menstruation (The Latin word for
"month" is menses). The cycle is defined as a succession of events
repeated regularly within a given period of time. So the occurrence of
menstruation at regular intervals is called the menstrual cycle. Generally, it
is repeated after every 28 days. However, the number of days is variable in
different individuals or within the same individual at different times.
The menstrual cycle can be
divided into four stages:
(a) Follicular stage
(b) Ovulation stage
(c) Luteal stage
(d) Menstrual stage
Follicular
Stage: One or sometimes more than one egg and follicles start
growing in size. The egg is surrounded by a follicle. The follicles arrange
themselves in layers. So a large cavity appears between the follicle cells and
the developing egg. Some of the follicle cells secrete estrogen. It affects the
growth of the soft tissues of the uterus. Primarily estrogen affects the
internal lining of the uterus. So the uterine tissue becomes spongy in nature.
Ovulation
stage: At about the midpoint of twenty-eight to thirty days
cycle, under the influence of Luteinizing hormone, ovulation takes place. The
ovum then passes to the fallopian tube (or oviduct).
Luteal
stage: After ovulation, continued LH stimulation causes the
follicle cells to secrete glandular cells. These glandular cells constitute the
corpus luteum. The maturing corpus luteum secretes progesterone. Progesterone
acts on the uterus by stimulating vascularization (blood vessel development),
glandular development, and glycogen accumulation in the endometrium. As a
result, the uterine epithelium becomes folded, and the endometrium becomes soft
and spongy.
Menstrual
stage: In a typical menstrual cycle, when a pregnancy has not
begun, the LH level falls. It causes the corpus luteum to begin to degenerate
by the 21st or 22nd day. As the corpus luteum is degenerating, its hormone
output decreases. The thickened spongy part of the uterus starts to break down,
and menstruation begins. During this phase, the broken-down soft tissues of the
uterus and a considerable amount of blood are released and pass out of the
vagina. The day on which menstruation begins is commonly designated as
"day one" of the menstrual cycle. Menstruation is a clear signal that
pregnancy has not begun. The menstrual stage lasts for 5-6 days. The cycle is
thus completed, and the uterus then enters into the next menstrual cycle.
Age
for Menstruation: It begins usually at the age of 12-14 in
females. It is variable.
Menopause:
Between the ages of approximately 43 and early 50 a woman loses her ability to
produce ova. The menstrual cycle ceases. Her breasts, genitalia and uterus all
decrease in size. Sexual desire may also be reduced.
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| Menstrual Cycle |
Role of Pituitary Gland
Gonadotropic:
The pituitary hormones that play an important role in the reproductive cycle in
females are commonly called gonadotropins. The most important gonadotropins
are:
(a) Follicle-stimulating
hormone.
(b) Luteinizing hormone.
Both these hormones affect
the ovary. FSH affects the developing oocyte. Oocyte is the cell that will give
rise to the ovum. The follicle also increases due to FSFI (Female Sexual
Function Index). The mature follicles are called Graafian follicles. LH at this
stage causes ovulation. The follicle cells surrounding the developing oocyte
secrete estrogen. So both FSH and LH cause the secretion of estrogen in an
indirect way. Estrogen causes structural and functional changes in the uterus.
Luteotropic
Hormone
It is secreted by the
pituitary gland. LTH is also called prolactin. It stimulates the development
and secretion of milk in the mammary gland. LTH also stimulates the corpus
luteum to secrete progesterone.
Role
of Placenta
Some cells of the placenta
become specialized. These cells secrete hormones. The hormones are like the
pituitary hormone in function.
Chorionic
Gonadotropin
This hormone is secreted
by the chorion of the placenta. Its function is similar to LH and LTH. Due to
CG, the activity of the corpus luteum is maintained within the ovary. So there
is secretion of progesterone for a considerable period. The placenta then
starts secreting its own progesterone. At this stage, the secretion of ovarian
progesterone ceases.
Functions:
Major functions of human estrogen and progesterone during pregnancy are:
Estrogen
(1) Growth of mammary
glands.
(2) Inhibits FSH release.
(3) Inhibits prolactin
release.
(4) Prevents infection in
the uterus.
(5) Increases the size of
uterine muscle cells.
(6) Increases the
sensitivity of myometrium to oxytocin.
Progesterone
(1) Growth of mammary
glands.
(2) Inhibits FSH release.
(3) Inhibits prolactin
release.
(4) Inhibits the
contraction of myometrium.
The hypothalamus produces
GnRH (gonadotropic-releasing hormone). GnRH stimulates the anterior pituitary
to produce FSH (follicle-stimulating hormone) and LH (luteinizing hormone). FSH
stimulates the follicle to produce estrogen, and LH stimulates the corpus
luteum to produce progesterone.
Estrogen and progesterone
affect the sex organs (e.g., uterus) and the secondary sex characteristics and
exert feedback control over the hypothalamus and the anterior pituitary.
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| Role of hormones on testes and ovary |
Test Tube Babies
Test tube babies refer to
the in vitro fertilization (IVF) technique, which involves fertilizing one or
more eggs outside the body in laboratory glassware and then transferring the
fertilized egg, known as pre-embryos, back into the uterus. This technique was
first developed in 1978 to help women with blocked oviducts, and it has since
become a widely used method for couples struggling with infertility. The
process of IVF typically involves stimulating the ovaries with fertility drugs
to produce multiple eggs, collecting mature eggs, fertilizing the egg in the
laboratory, culturing the pre-embryo, and finally transferring the embryo to
the uterus. IVF is a highly effective procedure and has helped millions of
couples around the world conceive and give birth to healthy babies.
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